This invention relates to fluoropolymer coating and casting compositions and films which may be derived from such compositions.
It is well known that perfluoroplastic coatings resulting from the simple deposition of an aqueous dispersion on a surface will tend to develop cracks while undergoing consolidation during drying or fusing operations if their thickness exceeds a certain maximum value, commonly referred to as the "critical cracking thickness". In the case of polytetrafluoroethylene, this thickness is generally expected to be on the order of 0.001 inches, for coatings applied to smooth horizontal surfaces under ideal conditions. For fluorinated ethylenepropylene (FEP) or perfluoroalkoxy modified polytetrafluoroethylene (PFA) the value is less than 0.0005 inches. When such dispersions are applied to vertical surfaces, as is often desirable or necessary in the continuous coating of substrates in web form and in the dip or spray coating of objects, the effective critical cracking thickness is substantially reduced, often to less than half of the ideal values. In the case of relatively uneven or textured planar substrates, e.g. woven glass, it is difficult in practice to apply coatings of uniform thickness, since the coating fluids, under the influence of gravitional or capillary forces, tend to flow as they seek a position of minimum potential energy. This leads inevitably to a variation in the depth of such fluids and a resultant variation in the thickness of coalesced solids derived therefrom. Cracks can then develop when the thickness of deposited solids exceeds the critical value. In the case of three dimensional objects, surface tension and rapidly rising viscosity associated with coalescence often results in beading of the coating fluids along edges or corners of the object, resulting in thick resin deposits susceptible to cracking.
For certain purposes, such as in the coating of textiles, it is often necessary or desirable to apply uniform and continuous films of polytetrafluoroethylene or other fluoroplastics in thicknesses greater than the critical cracking thickness. A number of techniques for accomplishing this objective are well-known in the art.
Relatively thick crack-free coatings can be achieved through multiple, consecutive applications of coating fluid, each of which results in solid deposition less than the critical cracking thickness. Each application preferably involves drying and fusing of the solids between subsequent applications of the coating fluid. In the nown production of coated textiles as by dip coating, this multiple coat build-up has shortcomings. The technique requires that the textile substrate to which the coating is applied be repeatedly subjected to the high temperatures required to melt fluoroplastics, i.e. above 650.degree. F. in the case of polytetrafluoroethylene. Such repeated heating to such high temperature can result in degradation of the physical properties of the reinforcement and create mechanical stress concentrations in the coating matrix.
In the case of PTFE applied from relatively viscous fluid formulations, the problems associated with critical cracking thickness are dealt with by accepting the presence of cracks in an unfused or semi-fused condition, and subsequently attempting to heal them by calendaring at a temperature sufficient to soften the resin, but insufficient to melt it. The inability to uniformly deliver pressure to the resin thus consolidated due to the presence of hard and relatively stiff textile reinforcements reduces the effectiveness of this approach and some degree of microcracking remains even in the finished, fused products. This method is, moreover, limited to the coating of flexible, planar substrates and is not applicable to the coating of irregular surfaces or objects.
While mineral fillers may be employed to increase the nominal critical cracking thickness, their presence is often undesirable for reasons related to the end-use of products so manufactured.
U.S. Pat. No. 2,681,324 to Hochberg describes the addition of large quantities of aqueous dispersions of polymeric materials, such as alkyl acrylates and blends threof, butyl methacrylate, styrene, styrene/butadiene copolymers, acrylonitrile, or butadiene/acrylonitrile copolymers, to polytetrafluoroethylene dispersions to form codispersions which can be used to apply relatively thick, crackfree poly tetrafluoroethylene coatings. These materials are thermally unstable at temperatures well below those required to fuse polytetrafluoroethylene and are for the most part fugitive in normal processes with attendant shrinkage of the coalescing film. U.S. Pat. No. 2,710,266, to Hochberg describes the addition of aqueous solutions of alkali metal silicates, for example sodium silicate, to polytetrafluoroethylene dispersions for the same purpose. While suitable for certain substrates, such compositions are largely unsuitable for the more commonly employed textile substrates, such as glass cloth.
Accordingly, it is an object of this invention to provide a fluid fluoropolymer coating composition which can be used to produce, with fewer applications of the fluid, relatively thick, crack-free coatings.
It is also an object of this invention to produce films, supported or unsupported, which are considerably thicker than those which can be readily produced from known formulations, and which may be formed as a planar sheet or a three-dimensional product, such as a glove, bladder, or other shape by dipping or spraying a removable mandrel.
It is a further object of the invention to provide fluoropolymer coating compositions which are process compatible with known fluoroplastics in that films formed therefrom can be thermally welded to them as well as to each other and to fluoroplastics, including perfluoroplastics.
It is also an object of this invention to provide fluid fluoropolymer coating compositions which may be useful which may be useful for the improvement of fluoropolymer-containing articles by ameliorating undesirable behavior stemming from mudcracking and limited build rate inherent to known compositions and methods.
It is another object of this invention to provide fluoropolymer coatings, composites or film properties the mechanical behavior of which can be controlled through selective formulations of fluoroplastics and fluoroelastomers.
It is yet another object of this invention to provide fluoropolymer coatings or films with physical, chemical and electrical characteristics different from those obtainable with known fluoropolymers.
It is finally an object of this invention to provide a fluoropolymer coating composition able to bridge the relatively substantial discontinuities associated with many textile substrates.